1. Field of the Invention
The present invention relates to a magnetic recording medium, particularly a magnetic recording medium obtained by coating magnetic coating as the dispersion of magnetic particulates and binder in the solvent over non-magnetic support. More specifically, the invention relates to a magnetic recording medium used for videotapes, audiotapes, floppy disks, data storage tapes for computers, whose electromagnetic transducing characteristic and durability are improved.
2. Description of the Related Art
Recently, with increasing performance of high vision VTR, digital VTR, etc., improvement in the characteristics of a magnetic recording medium has been demanded. Therefore, developments of evaporation-type magnetic recording medium which can provide electromagnetic transducing characteristic with higher output than coat-type one are more actively carried out.
However, evaporation-type magnetic recording medium have high surface energy on the magnetic surface, causing a problem of deterioration of durability due to friction, because they are formed by evaporating magnetic metals such as iron, cobalt and nickel on non-magnetic support under vacuum.
Moreover, evaporation process needs complicated, expensive equipments such as vacuum system and laser to evaporate metals and the productivity is low as compared with the other coat-type magnetic recording medium.
Thereby, developments of high performance coat-type magnetic recording medium have been actively studied. Recently, reduction of the size of magnetic powder used has been attempted because shortening recording wavelengths is promoted.
Under these circumstances, it is important to develop a binder resin to contain these more recently developed high performance particulates in a magnetic recording medium which combines the features of dispersibilty and durability.
Conventionally, common resins as binders for magnetic recording include vinyl chloride copolymers, polyurethane resin, cellulose resin (Japanese Patent Laid-Open No. 5-319777), phenoxy resin, polyacetal resin (Japanese Patent Laid-Open No. 5-329318) and are used alone or in combination.
Out of these resins, polyurethane resin has been variously studied so far because it has a wide range of properties and various functional groups can be introduced. Furthermore, in order to obtain high performance magnetic recording medium, improvement in solubility of various resin components and lowering of viscosities of resins allowing high dispersion of solid components are necessary.
For example, Japanese Patent Laid-Open No. 7-235044 disclosed that excellent characteristics of electromagnetic transduction is available by using the polyurethane resin with tertiary amine as polar group. Moreover, because reduction of the size of magnetic powder used has been attempted in recent years, low viscosity and dispersion of hard solid components are demanded. Therefore, for example, Japanese Patent Laid-Open No. 3-190983, Japanese Patent Laid-Open No. 3-203811 disclosed the method using the polyurethane resin of a certain composition into which alkylphosphine group is introduced.
Similarly, as the binder effective in dispersibility, Japanese Patent Laid-Open No. 7-50010 disclosed urethane urea using a given amine, which has been variously studied so far.
However, since every binder which has been studied so far contains large amounts of polar groups to improve the dispersibility, it is known that the viscosity of binder resins increases and the resin is hard to handle during manufacture, and resultantly sufficient effect frequently was not achieved.
Also concerning resin viscosity, such resins can not be well mixed with several binders and the thixotropy of a magnetic coating increases, so that high speed coating could not be performed with low productivity.